Rats (Rattus norvegicus) living in complex habitats with designed communication networks develop capacities for "hypothesis building" that facilitate more effective interactions with associates. Required acquisition of cooperative behavior reduces stress, even at above optimal population densities. Rats also exhibit the capacity to acquire non-experimenter-required cooperative and altruistic behaviors, as well as to physically modify access to routes of communications between segments of the environment inhabited by different subgroups. Such "hypothesis-building" activities have dual consequences of fostering longer duration, more complex, behaviors, and buffering the deleterious consequence of above optimal densities.